is formed. Here, \(G\)  is the gravitational constant, \(c\)  is the speed of light and \(\pi _{\text{rel}}\)  is the relative lens-source parallax (\(\pi _{\text{lens}}-\pi _{\text{source}}\) ). In the case of imperfect lens-source alignment, two images of the source are formed. As the lens passes between the source and observer the source images change brightness and position giving rise to transient photometric [1]}, [2]} and astrometric microlensing effects , [3]}, [4]}. In contrast to other methods, such as X-ray binary studies [5]} and gravitational wave searches [6]}, it remains the only one sensitive to isolated stellar-mass black holes.
